Dear HBase dev: In HBASE-21879, we redesigned the offheap read path: read the HFileBlock from HDFS to pooled offheap ByteBuffers directly, while before HBASE-21879 we just read the HFileBlock to heap which would still lead to high GC pressure.
After few months of development and testing, all subtasks have been resovled now except the HBASE-21946 (It depends on HDFS-14483 and our HDFS teams are working on this, we expect the HDFS-14483 to be included in hadoop 2.8.6 and after that the HBASE-21946 will get resolved). we think the feature is stable enough now and it's time to merge branch HBASE-21879 back to master now. We have designed 3 test cases to prove the performance improvment with HBASE-21879: 1. Disabled BlockCache, which means the cacheHitRatio is 0%; 2. CacheHitRatio~65%; 3. CachehHitRatio~100%; In our performance results, we can see that: the case#1 have an great performance improvement ( *throughput increased about 17%, heap allocation decreased about 95%, Young generaion size decreased about 81.7%*), that's because after HBASE-21879 all reads will allocate from pooled offheap bytebuffers and almost no heap allocation, while before HBASE-21879 the read path will create so many heap allocations. On the other hand, from the testing results of case#2 and case#3 we can also see that: *As the cacheHitRatioincreasing, the difference between before-HBASE-21879 and after-HBASE-21879 will decrease, when cacheHitRatio is 100%, they almost have no much difference in both throughput and latency.* For more details please see the document. Thanks Anoop/Ram/DuoZhang/Stack/GuanghaoZhang very much for your meticulous work (Suggession, discussion, patch reviewing, doc reviewing etc). Please vote  +1  +0/-0  -1 Do not merge the branch back because... Thanks. Any suggestions are welcomed.  https://issues.apache.org/jira/browse/HBASE-21879  https://issues.apache.org/jira/browse/HBASE-21946  https://issues.apache.org/jira/browse/HDFS-14483  https://docs.google.com/document/d/1xSy9axGxafoH-Qc17zbD2Bd--rWjjI00xTWQZ8ZwI_E